Electrical Conductivity Of Earth Research Articles

Analysis of the Geoelectric Field in Sweden Over Solar Cycles 23 and 24: Spatial and Temporal Variability During Strong GIC Events

AbstractGeomagnetic storms can produce large perturbations on the Earth magnetic field. Through complex magnetosphere‐ionosphere coupling, the geoelectric field (E) and geomagnetic field (B) are highly perturbed. The E is the physical driver of geomagnetically induced currents. However, a statistical study of the E in Sweden has never been done before. We combined geomagnetic data from the International Monitor for Auroral Geomagnetic Effects network in Northern Europe with a 3‐D structure of Earth's electrical conductivity in Sweden as the input of a 1‐D model to compute the E between 2000 and 2018. Northwestern Sweden presents statistically larger E magnitudes due to larger |dB/dt| variations in the north than in the south of Sweden and relative lower conductivity in the west compared to central and eastern Sweden. In contrast, the 15 strongest daily maximum |E| events present more frequently a maximum magnitude in central Sweden (62.25°N) and their relative strengths are not the same for all latitudes. These results highlight the different regional response to geomagnetic storms, which can be related to ground conductivity variability and the complex magnetosphere‐ionosphere coupling mechanisms.

3D Modeling of Geomagnetically Induced Currents in Sweden—Validation and Extreme Event Analysis

AbstractRosenqvist and Hall (2019), https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018SW002084 developed a proof‐of‐concept modeling capability that incorporates a detailed 3D structure of Earth's electrical conductivity in a geomagnetically induced current estimation procedure (GIC‐SMAP). The model was verified based on GIC measurements in northern Sweden. The study showed that southern Sweden is exposed to stronger electric fields due to a combined effect of low crustal conductivity and the influence of the surrounding coast. This study aims at further verifying the model in this region. GIC measurements on a power line at the west coast of southern Sweden are utilized. The location of the transmission line was selected to include coast effects at the ocean‐land interface to investigate the importance of using 3D induction modeling methods. The model is used to quantify the hazard of severe GICs in this particular transmission line by using historic recordings of strong geomagnetic disturbances. To quantify a worst‐case scenario GICs are calculated from modeled magnetic disturbances by the Space Weather Modeling Framework based on estimates for an idealized extreme interplanetary coronal mass ejection. The observed and estimated GIC based on the 3D GIC‐SMAP procedure in the transmission line in southern Sweden are in good agreement. In contrast, 1D methods underestimate GICs by about 50%. The estimated GICs in the studied transmission line exceed 100 A for one of 14 historical geomagnetic storm intervals. The peak GIC during the sudden impulse phase of a “perfect” storm exceeds 300 A but depends on the locality of the station as the interplanetary magnetic cloud hits Earth.

Observing and modeling the effects of production infrastructure in electromagnetic surveys

Electromagnetic (EM) methods are among the original techniques for subsurface characterization in exploration geophysics because of their particular sensitivity to the earth electrical conductivity, a physical property of rocks distinct yet complementary to density, magnetization, and strength. However, this unique ability also makes them sensitive to metallic artifacts — infrastructure such as pipes, cables, and other forms of cultural clutter — the EM footprint of which often far exceeds their diminutive stature when compared to that of bulk rock itself. In the hunt for buried treasure or unexploded ordnance, this is an advantage; in the long-term monitoring of mature oil fields after decades of production, it is quite troublesome indeed. Here we consider the latter through the lens of an evolving energy industry landscape in which the traditional methods of EM characterization for the exploration geophysicist are applied toward emergent problems in well-casing integrity, carbon capture and storage, and overall situational awareness in the oil field. We introduce case studies from these exemplars, showing how signals from metallic artifacts can dominate those from the target itself and impose significant burdens on the requisite simulation complexity. We also show how recent advances in numerical methods mitigate the computational explosivity of infrastructure modeling, providing feasible and real-time analysis tools for the desktop geophysicist. Lastly, we demonstrate through comparison of field data and simulation results that incorporation of infrastructure into the analysis of such geophysical data is, in a growing number of cases, a requisite but now manageable step.

Anomalous Behavior of Viscosity and Electrical Conductivity of MgSiO3 Melt at Mantle Conditions

AbstractSilicate melts have served as transport agents in the chemical and thermal evolution of Earth. Molecular dynamics simulations based on a deep neural network potential trained by ab initio data show that the viscosity of MgSiO3 melt decreases with increasing pressure at low pressures (up to ∼6 GPa) before it starts to increase with further compression. The melt electrical conductivity also behaves anomalously; first increasing and then decreasing with pressure. The melt accumulation implied by the viscosity turnover at ∼23 GPa along mantle liquidus offers an explanation for the low‐velocity zone at the 660‐km discontinuity. The increase in electrical conductivity up to ∼50 GPa may contribute to the steep rise of Earth's electrical conductivity profiles derived from magnetotelluric observations. Our results also suggest that small fraction of melts could give rise to detectable bulk conductivity in deeper parts of the mantle.

MAGLAB: A computing platform connecting geophysical signatures to melting processes in Earth's mantle

Decompression melting of the upper mantle produces magmas and volcanism at the Earth's surface. Experimental petrology demonstrates that the presence of CO2 and H2O enhances peridotite melting anywhere within the upper mantle down to approximately 200–300 km depth. The presence of mantle melts with compositions ranging from carbonate-rich to silicate-rich unavoidably affects the geophysical signals retrieved from Earth's mantle. Geochemical investigations of erupted intraplate magmas along with geophysical surveys allow for constraining the nature and volume of primary melts, and a sound formalism is required to integrate these diverse datasets into a realistic model for the upper mantle including melting processes. Here, we introduce MAGLAB, a model developed to calculate the composition and volume fraction of melts in the upper mantle, together with the corresponding electrical conductivity of partially molten mantle peridotites at realistic pressure-temperature conditions and volatile contents. We use MAGLAB to show how the compositions of intraplate magmas relate to variations in lithosphere thickness. Progressive partial melting of a homogeneous peridotitic mantle source can in theory create the diversity of compositions observed among the spectrum of intraplate magma types, with kimberlite melts beneath thick continental shields, alkaline magmas such as melilitite, nephelinite and basanite beneath thinner continents and relatively old plus thick oceanic lithospheres, and ‘regular’ basalts beneath the youngest and thinnest oceanic lithospheres as well as beneath significantly thinned continental lithospheres. MAGLAB calculations support recent experimental findings about the role of H2O in the upper mantle on producing primary kimberlitic melts in addition to CO2. We demonstrate the robustness of MAGLAB calculations by reproducing the compositions of erupted melts as well as associated mantle electrical conductivities beneath the Society hotspot in the Pacific Ocean. A comparison of our simulations with magnetotelluric surveys at various oceanic settings shows that the heterogeneities in electrical conductivity of Earth's upper mantle are related to variations in volatile content via the presence of small (generally <<1 wt%) and heterogeneously distributed fractions of CO2-H2O-bearing melts.

Modified GIC Estimation Using 3‐D Earth Conductivity

AbstractGeomagnetically induced currents (GICs) are quasi‐direct current (DC) electric currents that flow in technological conductors during geomagnetic storms. Extreme GICs are hazardous to man‐made infrastructure. GICs enter and exit the technological systems, such as the electric power grid, at grounding points, and their magnitudes depend on the currents that flow underground. They are, therefore, a function of the Earth's electrical conductivity, represented at ground level as Earth impedances, as well as the resistance parameters of the power network. Traditional GIC estimation practices are based on Earth impedances obtained from laterally homogeneous or piecewise layered‐Earth models. We refer to these methods, collectively, as the 1‐D approximation. However, GIC hazard mitigation can be improved with more accurate GIC modeling that takes the spatially heterogeneous Earth's conductivity into account. Here, we propose a modified approximation for GIC estimation that is very similar to the 1‐D approximation but is instead derived from empirical 3‐D Earth impedances. Our formulation sets up the computation of static, frequency‐dependent power line telluric response functions, which, once computed, may be considered part of the power grid system model. These response functions may then be used for historical scenario analysis of GIC hazards and for simplified real‐time, albeit approximate, GIC estimation in a power grid. This modest modification to the simpler local field formulation approach avoids real‐time integration of geoelectric fields along power lines while taking the realistic 3‐D Earth into account in a rigorous manner. Once implemented, the method provides a power grid operator with the benefits of convenience and computational speed for a first look real‐time operational GIC hazard assessment. We estimate that the proposed modified 3‐D GIC modeling approach produces GIC values that are well within 50% of those obtained with the full‐scale power line integration of spatially variable geoelectric fields, for storms comparable in scale to the 2003 Halloween storm, all geological structures, and power lines located in the contiguous United States and other low‐ to middle‐latitude regions.

Differential Magnetometer Measurements of Geomagnetically Induced Currents in a Complex High Voltage Network

AbstractSpace weather poses a hazard to grounded electrical infrastructure such as high voltage (HV) transformers, through the induction of geomagnetically induced currents (GICs). Modeling GICs requires knowledge of the source magnetic field and the Earth's electrical conductivity structure, in order to calculate the geoelectric fields generated during magnetic storms, as well as knowledge of the topology of the HV network. Direct measurement of GICs at the ground neutral in substations is possible with a Hall effect probe, but such data are not widely available. To validate our HV network model, we use the differential magnetometer method (DMM) to measure GICs in the 400 kV grid of Great Britain. We present DMM measurements for the 26 August 2018 storm at a site in eastern Scotland with up to 20 A recorded. The line GIC correlates well with Hall probe measurements at a local transformer, though they differ in amplitude by an order of magnitude (a maximum of 2 A). We deployed a long‐period magnetotelluric (MT) instrument to derive the local impedance tensor which can be used to predict the geoelectric field from the recorded magnetic field. Using the MT‐derived electric field estimates, we model GICs within the network, accounting for the difference in magnitude between the DMM‐measured line currents and earth currents at the local substation. We find that the measured line and earth GICs match the expected GICs from our network model, confirming that detailed knowledge of the complex network topology and its resistance parameters is essential for accurately computing GICs.

Surface MMR enhanced 3D inversion using model reconstruction strategy

Previous inversions of surface magnetometric resistivity (MMR) could obtain only the ratio, not the absolute value of earth electrical conductivity. In this paper, instead of solving Poisson's equation, the magnetic fields generated by low frequency alternating current(AC) line are looked as MMR data and calculated from electric-type Helmholtz's equation in frequency domain, so that the absolute conductivity could be taken into account. Forward modeling scheme is adapted from staggered-grid finite-difference method. Inversion algorithm is non-linear conjugate gradient (NLCG). The magnetic field perpendicular to line source is selected as observed data. To improve depth resolution, we introduce a model reconstruction strategy based on reference model. In the strategy, the upper portion of the resultant model from previous inversion is replaced by a homogeneous layer to construct a new model to trigger next inversion. The thickness of the homogeneous layer is called reconstruction depth, it is equal to the depth where the models difference between last two iterations after the run of the ith inversion is almost none-zero. The reconstruction could be applied until the model difference is almost zero or the difference of model differences between two adjacent inversions is less than a predefined threshold. Inversion results from synthetic and real data have proved that the proposed multi-reconstruction inversion strategy is able to obtain absolute resistivity, as well as enhance the deep resolution of surface MMR data inversion.

Predicting Global Ground Geoelectric Field With Coupled Geospace and Three‐Dimensional Geomagnetic Induction Models

AbstractWe forecast the global effects of space weather on the geoelectric and geomagnetic fields using a novel combination of methods. We use a realistic three‐dimensional (3‐D) model of Earth's electrical conductivity and a realistic representation of magnetospheric and ionospheric current systems. Our scheme involves the following steps: (1) We run a global magnetohydrodynamic model of the magnetosphere coupled to an electrostatic model of the ionosphere. (2) We calculate a global time series of the ground magnetic field resulting from the ionospheric, field‐aligned, and magnetospheric currents of the global magnetohydrodynamic model. (3) We approximate this external field by an equivalent source current flowing in a thin shell above Earth. (4) We calculate a global time series of geoelectric and geomagnetic fields from the equivalent current and a 3‐D conductivity model of Earth that also takes into account the coast effect due to large horizontal conductivity gradient. We verify our implementation by comparing the results against known analytic and numeric solutions, and then apply our scheme to the geomagnetic storm of 14 and 15 December 2006. In particular, we show that accounting for 3‐D structure of Earth's conductivity results in significantly enhanced geoelectric field at large lateral gradients of conductivity, especially in coastal regions, both at middle and high latitudes. In the studied geomagnetic storm the largest values of 3‐D geoelectric field are detected at high latitudes reaching 2.5 V/km and the 3‐D effect extends inland by a few hundred kilometers.

Estimating the power-law distribution of Earth electrical conductivity from low-frequency, controlled-source electromagnetic responses

Estimating the power-law distribution of Earth electrical conductivity from low-frequency, controlled-source electromagnetic responses

VLF waves at satellite altitude to investigate Earth electrical conductivity

AbstractAt and near the Earth surface, electromagnetic (EM) fields radiated from VLF transmitters are commonly used in geological exploration to determine the shallow Earth conductivity structure. Onboard satellites such as DEMETER, the electric and magnetic sensors detect the VLF signal in altitude. While we know for surface measurement that the VLF EM field recorded at some distance from the transmitter is a function of the ground conductivity, we do not know how this relationship changes when the field is measured at satellite altitude. Here we study the electromagnetic field radiated by a vertical electric dipole located on the Earth surface in the VLF range and measured at satellite altitude in a free space. We investigate the EM field as function of distance from the source, the height above the Earth surface, and the electrical conductivity of the Earth. The mathematical solution in altitude has more severe numerical complications than the well‐known solutions at or near the Earth surface. We test most of the solutions developed for the latter case and found that direct summation was best at several hundred kilometers above the Earth. The numerical modeling of the EM field in altitude shows that the field remains a function of Earth conductivity. The dependence weakens with altitude and distance from the transmitter. It remains more important for the electric radial component.

Determination of the 1-D distribution of electrical conductivity in Earth’s mantle from Swarm satellite data

We present an inversion scheme to recover the (1-D) depth profile of mantle conductivity from satellite magnetic data, which takes into account 3-D effects arising from the distribution of oceans and continents. The scheme is based on an iterative inversion of C-responses, which are estimated from time series of the dominating external (inducing) and internal (induced) spherical harmonic coefficients of the magnetic potential due to a magnetospheric source. These time series will be available as a Swarm Level-2 data product. We verify our approach by using synthetic, but realistic time series obtained by simulating induction due to a realistic magnetospheric source in a 3-D “target” conductivity model of the Earth. This model contains not only a laterally heterogeneous layer representing oceans and continents, but also 3-D inhomogeneities in the mantle. The inversion for mantle conductivity is initiated with a uniform conductivity model. Convergence is reached within a few iterations. The recovered model agrees well with the laterally averaged target model, although the latter comprises large jumps in conductivity. Our 1-D inversion scheme is therefore ready to process Swarm data.

Determination of the 3-D distribution of electrical conductivity in Earth’s mantle from Swarm satellite data: Frequency domain approach based on inversion of induced coefficients

Mapping the three-dimensional (3-D) electrical conductivity of Earth’s mantle has been identified as one of the primary scientific objectives for the Swarm satellite mission. We present a 3-D frequency domain inversion scheme to recover mantle conductivity from satellite magnetic data. The scheme is based on an inversion of time spectra of internal (induced) spherical harmonic coefficients of the magnetic potential due to magnetospheric sources. Time series of internal and external (inducing) coefficients, whose determination is a prerequisite for this formulation, will be available as a Swarm Level-2 data product. An iterative gradient-type (quasi-Newton) optimization method is chosen to solve our 3-D non-linear inverse problem. In order to make the inversion tractable, we elaborate an adjoint approach for a fast and robust calculation of the data misfit gradient. We verify our approach with synthetic, but realistic time spectra of internal coefficients, obtained by simulating induction due to a realistic magnetospheric source in a 3-D conductivity model of the Earth. In these model studies, both shape and conductivity of a large-scale conductivity anomaly in the mid-mantle are recovered very well. The inversion scheme also shows to be robust with respect to noise and is therefore ready to process Swarm data.

Electrical Conductivity of Mantle Minerals: Role of Water in Conductivity Anomalies

The electrical conductivity of Earth's mantle has recently become an interesting topic across diverse Earth science communities. Many electrical conductivity data of mantle phases have been accumulated through the development of high-pressure experiments. These data will provide information on valence states, water concentration, Fe concentration, oxygen fugacity, and the connectivity of the conductive phase in geological materials such as minerals, melts, and rocks. Although several groups have measured the electrical conductivity of mantle materials at high pressure, they have provided inconsistent results, especially with regard to the effect of water. Thus, it is timely to review the problems underlying experimental techniques. We discuss the current understanding of the effect of water on the electrical conductivity of nominally anhydrous mantle minerals, with some speculation on the form of volatile components in Earth's interior. Finally, we consider the role of water in major conductivity anomalies observed in the upper mantle and transition zone.

Constraining the composition and thermal state of the mantle beneath Europe from inversion of long‐period electromagnetic sounding data

We reexamine the problem of inverting C responses, covering periods between 1 month and 1 year collected from 42 European observatories, to constrain the internal structure of the Earth. Earlier studies used the C responses, which connect the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic variations, to obtain the conductivity profile of the Earth's mantle. Here, we go beyond this approach by inverting directly for chemical composition and thermal state of the Earth, rather than subsurface electrical conductivity structure. The primary inversion parameters are the composition of the Earth's mantle within the system CaO‐FeO‐MgO‐Al2O3‐SiO2 and geotherm. Given these parameters, we calculate mineral modes at the prevailing physical conditions and combine these with laboratory‐based models for the conductivity of individual minerals to estimate the bulk Earth electrical conductivity structure from which C responses are calculated. To further constrain the radial density profile, we also invert for mass and moment of inertia. The results agree with earlier geophysically derived conductivity and seismic velocity models and confirm that inversion of geophysical data for compositional parameters, planetary composition, and thermal state is feasible. The inversion indicates most probable lower mantle geothermal gradients of ∼0.58 K/km, core mantle boundary temperatures of ∼2900°C, bulk Earth molar Mg/Si ratios of ∼1.1, intermediate between the chondrite and pyrolite models, and no significant change in mantle composition across the 670‐km seismic discontinuity.

Practical Magnetotellurics

Practical Magnetotellurics lives up to its title and nicely fills a big hole in the literature on the subject of the magnetotelluric (MT) method, a means by which the geological and physical properties of the crust and mantle may be studied using electrical conductivity images.In an MT sounding passive time series, measurements of the Earth's naturally occurring magnetic and electric fields are used to generate an electromagnetic impedance of the subsurface as a function of frequency, which may in turn be interpreted in terms of Earth electrical conductivity. Because short‐period (higher frequency) energy is absorbed more rapidly in conductive rocks than longer‐period MT fields, the method has an intrinsic depth resolution, which for appropriate frequencies can span the range from a few meters at the surface all the way to the lower mantle. Electrical conductivity is a strong function of porosity, mineralogy, and temperature, so applications of the MT method include petroleum and mineral exploration, groundwater and environmental studies, structural geology and understanding the geodynamical state of the planet.

Observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity

Currents induced in Earth by temporal variations in the external magnetic field have long been used to probe mantle electrical conductivity, but almost exclusively from sparsely distributed land observatories. Satellite‐borne magnetometers, such as flown on Magsat, Ørsted, and Champ, offer the prospect of improved spatial coverage. The approach we have taken is to isolate induction by harmonic Dst (“disturbance storm time”) excitation of the magnetospheric ring current in satellite magnetic measurements: this is done by removing the magnetic contributions of the main (core) magnetic field, the crustal magnetic field, and ionospheric fields (cause of the daily variation) using Sabaka et al.'s [2000, 2002]CMP3 comprehensive model. The Dst signal is then clearly evident in the midlatitude satellite passes lower than 50 degrees geomagnetic latitude. At higher latitudes, auroral and field aligned currents contaminate the data. We fit the internal and external components of the Dst signal for each equatorial pass, exploiting the fact that the geometry for the internal and external components is different for the azimuthal and radial vector components. The resulting timeseries of internal and external field variations shows that the Dst signals for the dawn passes are half those of the dusk passes. The sum of equatorial external and internal components of the field averaged over dawn and dusk passes provides an excellent estimate for the Dst index, and may in fact be superior when used as a proxy for the purposes of removing induced and magnetospheric fields from satellite magnetic data. We call this estimate satellite Dst. Cross spectral analysis of the internal and external timeseries shows both greater power and higher coherence in the dusk data. We processed the transfer function between internal and external dusk timeseries to provide globally‐averaged, frequency dependent impedances that agree well with independently derived estimates. We estimate Earth's radial electrical conductivity structure from these impedances using standard regularized inversion techniques. A near‐surface conductor is required, of thickness less than 10 km with a conductivity‐thickness product almost exactly that of an average Earth ocean. Inversions suggest that an increase in conductivity at 440 km depth, predicted by recent laboratory measurements on high pressure phases of olivine, is not favored by the data, although, as in previous studies, the 670 km discontinuity between the upper and lower mantle is associated with a two orders of magnitude jump in conductivity. A new feature in our inversions is a further increase in lower mantle conductivity at a depth of 1300 km. A global map of the internal (induced) component of the magnetic field provides a qualitative estimate of three‐dimensional (3‐D) variations in Earth electrical conductivity, demonstrating graphically that the satellite data are responsive to lateral variations in electrical conductivity caused by the continents and oceans.

Induction arrows from offshore floating magnetometers using land reference data

SUMMARY Induction arrows are a traditional output of magnetovariational experiments, and represent transfer functions used quantitatively in the inversion of geomagnetic depth sounding data to give Earth electrical conductivity structure. In this paper, a technique is tested in which ‘total-¢eld’ variations are combined with horizontal-¢eld data recorded simultaneously at remote stations in order to derive induction arrows. The method is ¢rst demonstrated using total-¢eld data recorded on land by an aeromagnetic base station, and then applied to data obtained from magnetometers £oated oishore on the sea surface. The £oating magnetometers were deployed in two con¢gurations: one tethered to the sea£oor; the second free-£oating.

Electrical conductivity of olivine, wadsleyite, and ringwoodite under upper-mantle conditions

Geophysical models show that electrical conductivity in Earth's mantle rises about two orders of magnitude through the transition zone in the depth range 410 to 660 kilometers. Impedance measurements obtained on Mg1.8Fe0.2SiO4 olivine, wadsleyite, and ringwoodite at up to 20 gigapascals and 1400 degreesC show that the electrical conductivities of wadsleyite and ringwoodite are similar and are almost two orders of magnitude higher than that of olivine. A conductivity-depth profile to 660 kilometers, based on these laboratory data, shows a conductivity increase of almost two orders of magnitude across the 410-kilometer discontinuity; such a profile favors a two-layer model for the upper mantle. Activation enthalpies of 1.2 to 1.7 electron volts permit appreciable lateral variations of conductivity with lateral temperature variations.

Large‐scale electric field measurements on the Earth's surface: A review

There exist only a few reported measurements of quasi‐stationary (near dc) electric potentials over very large spatial scales (hundreds of kilometers or more) on the Earth's surface. Such measurements have typically been made using unpowered submarine telecommunications cables. The measurements pose unique experimental challenges and require careful procedures to avoid data contamination by electrode contact potentials and local ground currents. In addition, there are possible interpretational problems from pervasive, poorly understood, low‐frequency electric fields induced by ocean water motion through the Earth's stationary magnetic field. Nevertheless, estimates of the magnitude of the electric field computed from large‐scale potential difference measurements, made principally to date in the Pacific Ocean, can be used to place a limit on the size of the toroidal magnetic field at the core‐mantle boundary under certain conditions on the Earth's electrical conductivity profile. Thus, large‐scale electric potential measurements can serve as an adjunct probe of the Earth's dynamo process in addition to measurements of the poloidal magnetic field and its secular changes made at and above the surface of the Earth. A review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in magnitude.